Many types of retroreflective elements are known, including prismatic designs incorporating one or more "cube-corners." Generally, the prismatic elements are closely packed together when they are assembled into articles or sheeting, with no non-retroreflective portion between the elements. This close packing improves retroreflective performance by increasing the number of retroreflective elements per unit area of article or sheeting.
However, in some cases the close packing of the prismatic elements is not desired. For example, U.S. Pat. No. 4,775,219 (Appeldorn et al.), at column 7 lines 20-33, suggests that flat surfaces may be included between the prismatic elements disclosed in the patent so as to, for example, allow light from an automobile tail lamp to be directly transmitted through a retroreflective tail light reflector. This patent also indicates that "other structure" may be included between the intersection of the grooves that form the prismatic elements, but does not describe any examples. The prismatic elements in the patent are characterized by being formed by sets of intersecting V-shaped grooves, at least one set of grooves having groove side angles that are not constant for that set. The groove side angle is the angle between the groove side and a plane extending parallel to the length of the groove and perpendicular to the plane defined by the bottom edges of the three intersecting sets of V-shaped grooves
Similarly, U.S. Pat. No. 4,588,258 (Hoopman), at column 6, line 53 to column 7, line 21, discloses a cube-corner design in which non-retroreflective areas between the cube-corner elements may be provided by separating the elements from each other The separation of elements is one way to avoid removal of some edges of the cube-corner elements by a grooving tool used to construct the elements. Light may pass through these non-retroreflective areas, if a transparent or translucent article or sheeting base is used.
U.S. Pat. No. 3,924,929 (Holmen et al.) is representative of cube-corner designs in which non-retroreflective areas between cube-corner elements are provided and defined by what are designated as "free edges of septa." The septa are portions of the cube-corner material that have edges raised above, i.e. "free" from, the retroreflective portions of the material. At column 4, lines 38-54, and as shown in FIG. 3, Holmen et al. teach that the septa must be displaced away from the bases of the cube-corner elements, toward the trihedral corners, by a distance greater than the perpendicular elevation of the corners above the bases. Then, when a backing layer is attached to the septa at the free edges, an air space fully surrounding the cube-corner elements is created between the backing layer and those elements. At column 3, line 44 to column 4, line 2, they teach that the area of contact between an individual septum and the backing layer must be large enough to form a good bond, but the total area of all the septa must not unduly affect the overall retroreflective performance of the article. Again, light may pass through the non-retroreflective septa if they are made from a transparent or translucent material.